The present invention relates to an automatic gain control apparatus for use in a receiver of a radio communication system or the like, and more particularly to an automatic gain control apparatus using a digital control technique.
Automatic gain control apparatuses are extensively used in the receivers of radio communication systems or the like, and are intended to control the gain of the received signals, which may undergo amplitude variation by fading or other causes on the radio transmission line, so as to maintain a constant amplitude. Many recent control apparatuses use a digital, instead of analog, control technique. An automatic gain control apparatus using the digital control technique is mainly composed of an automatic gain controlled (AGC) circuit (or AGC amplifier), an analog/digital (A/D) converter, a digital AGC controller and a digtal/analog (D/A) converter. In this apparatus, the output of the AGC circuit is converted by the A/D converter into a digital signal, then by the digital AGC controller into a digital control signal, and further by the D/A converter into an analog control signal, which is supplied to the AGC circuit. The AGC circuit varies its gain in response to the analog control signal, and achieves control to maintain the output of the AGC circuit at a constant amplitude.
In such an automatic gain control apparatus, as will be described in detail hereinafter, if the numbers of bits in the A/D converter and the D/A converter are small and the output amplitude variability of the AGC circuit is great, the output amplitude control by the AGC circuit will be rough, and the output of the AGC circuit will fail to be constant, resulting in amplitude modulated signals. On the other hand, if the output amplitude is to be finely controlled, the aforementioned converters will have to be able to handle greater numbers of bits, which would mean higher costs for the converters and a more complex composition of the digital AGC controller. Moreover, this would also entail finer increments and decrements in the control for the digital control signals in the digital AGC controller, resulting in the disadvantage that the controller would be unable to follow abrupt variations in input amplitude caused by fading or some other factor.